Control circuit



Feb. '18, 1947. J BlCHSEL 2,416,144

CONTROL CIRCUIT Filed Aug. l5, 1944 2 Sheets-Sheet 2 I I I I I I I I *1I 2 I x I I I g I I I C I I I g l I I I I I l 253 234 L237 g :Z-35 w 1 Iv, I I I {I I I I R I I I I I I I I WITNESSES: I INVENTOR I 7 I HarryJfiz'c/wel.

Patented Feb. 18, 1947 CONTROL CIRCUIT Harry J. Bichsel, Forest Hills,Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh,Pa., a corporation of Pennsylvania Application August 15, 1944, SerialNo. 549,566

3 Claims. 1

This invention relates to an electronic control circuit and hasparticular relation to the control of resistance welding operations.

In connection with a resistance welding operation, it is generallybelieved that the applica tion of an additional pressure to the materialbeing welded at some time after initiation of the flow of weldingcurrent but during the welding period, results in an improved weld. Thisprocess involving the supplying of the additional pressure is known asforging.

Forging appears to be particularly important in the difiicult operationof spot Welding materials having a narrow plastic range and high thermalconductivity, such as aluminum. While generally satisfactory welds maybe made with such materials using a system known as capacitor dischargeresistance welding which provides a pulse of accurately measured weldingcurrent, many welding engineers agree that forging can be instrumentalin producing improved crack-free welds.

In most welding machines now on the market a fluid pressure system isassociated with the welding electrodes whereby the material to be weldedis clamped between the electrodes with a predetermined pressure. Forgingmay be accomplished with such an arrangement by operating a solenoidvalve in the pressure system to increase the pressure rapidly by a givenamount at a particular instant during the period in which weldingcurrent flows. However, considerable difiiculty has been encountered inattempting to operate the solenoid valve at the right instant.

It has been found that the instant of application of the additionalforging pressure in welding materials up to .125 of an inch thick shouldbe adjustable from .008 to .192 of a second after the initiation of thewelding current, the duration of the welding current, of course, alsobeing adjustable for materials of different thicknesses. Investigationsalso indicate that the instant of application of the pressure should notvary from that desired by more than. plus or minus .0016 of a second.

These time periods and the permissible error are exceedingly small whenthe nature of the operation is considered. It has been discovered thatthere is an appreciable time delay between the instant of energizationof the coil of the solenoid valve and the opening of the valve to permitthe application of an increased pressure. With many valves the timedelay is greater than the time permissible, at a minimum setting,between the initiation of the flow of welding current and theapplication of the increased pressure. For this reason it becomesnecessary to energize the coil of the solenoid valve in some casesbefore the initiation of the flow of welding current and in other casesafter the initiation of the flow of welding current. Moreover, sincesimultaneous operation of two contactors on a single relay of thestandard designs cannot be obtained with any degree of regularity, theuse of such relays in directly controlling the two operations must beavoided.

It is, accordingly, an object of my invention to provide new andimproved control apparatus for resistance welding.

Another object of my invention is to provide an extremely accuratetiming system for use in efiecting two different operations.

A further object of my invention is to provide resistance weldingapparatus with a control system whereby an accurately timed forgingoperation may be obtained.

A more specific object of my invention is to provide an extremelyaccurate control system for use in initiating the flow of weldingcurrent in a resistance welding machine and thereafter initiating theapplication of an increased pressure on the material to be welded at apreselected instant during the flow of welding current.

Other objects of my invention will become apparent upon reading thefollowing description of a specific embodiment of my invention inconnection with the accompanying drawings, in which:

Figure 1 is a circuit diagram illustrating an embodiment of myinvention; and

Fig. 2 is a graph illustrating the relationship with respect to timebetween various operations of the circuit of Fig. 1.

A capacitor discharge resistance welding system is shown in Fig. 1. Itincludes a main adjustable capacitor 3, which may be replaced by a bankof capacitors if desired, connected to be charged from alternatingcurrent supply lines 5 and 6 through a charging circuit 1 and to bedischarged through the primary winding 9 of a Welding transformer l lThe ends of secondary winding l3 of the transformer l I are connected todifferent ones of a pair of welding electrodes l5 and I! which areadapted to be moved relative to each other to clamp the material 19 tobe welded therebetween The advantages and operation of the charging anddischarging circuits are described in more detail in the copendingapplication of Hughes and Burgwin, Serial No. 511,575,

3 November 24, 1943, and assigned to the Westinghouse Electric iizManufacturing Company.

The charging circuit i receives its energy from an auxiliary transformer2| connected to lines 5 and ii and includes a pair of rectifier valves23 and 25 of the arc-like type, preferably thyratrons. The valves 23 and25 are connected to form a full wave rectifier between the secondarywinding 2'! of transformer 2! and the main capacitor 3 whereby directcurrent is supplied to charge the main capacitor with one plate 29positive and the other plate 3! negative.

Plate 29 of the main capacitor 3 is connected to one side of the primarywinding 9 of the welding transformer H. The other side of the primarywinding 9 is connected to the other plate 3! through a pair of inverselyconnected i nitrons 33 and 35. The cathode 31 of ignitron 33 isconnected to the originally negative plate SI of the main capacitor 3and the anode 39 is connected to the originally positive plate 28through the primary winding 9. Consequently, when ignitron 33 becomesconductive, the capacitor 3 is discharged through the primary winding 9and ignitron.

An electric discharge valve ii of the arc-like type, preferably athyratron, which is hereinafter designated as the first firing valve, isconnected in series with a current limiting resistor 33 between theanode 39 and the ignition electrode 45 of the ignitron 33. When thefirst firing valve 4! is rendered conductive, current starts to fiow inthe circuit extending from the plate 29 of the main capacitor 3 throughthe primary winding 9, the resistor 33, first firing valve ii, theignition electrode and cathode 31 of the ignitron 33 to the other plate3! of the capacitor. This ignition current renders the ignitron 33conductive and discharge of the main capacitor 3 is effected.

The control circuit of the first firing valve ii may be traced from itscathode 4'! through a source of biasing potential such as battery 3d,the secondary winding 5! of an impulse transformer 53 and a gridresistor 55 to the grid 5'8 of the first firing valve. The biasingpotential supplied from battery 59 normally maintains the first firingvalve id non-conductive. However, a potential may be impressed throughthe transformer 53 in a manner described hereinafter to overcome thebiasing potential and render the first firing valve conductive.

When the precharged main capacitor 3 is discharged, the dischargecurrent fiowing through the inductive primary winding 9 builds up amagnetic field. As the discharge of the main capacitor is completed, theresulting collapse of the magnetic field releases some of the energystored in the transformer I! and the secondary winding circuit to causethe main capacitor to be charged inversely with plate 3! positive andplate 29 negative. The anode 58 of ignitron 35 is connected to the plate3| while the cathode 59 is connected through the primary winding 9 tothe plate 29. Consequently, if ignitron 35 is then rendered conductive,the inverse charge on the main capacitor 3 is discharged through theignitron 35 and the primary winding 9.

Another electric discharge valve-6| of the arclike type, preferably athyratron, and hereinafter designated the second firing valve, isconnected in series with a current limiting resistor 63 between theanode 53 and the ignition electrode 65 of the ignitron 35. The grid 61of the second firing valve 8! is connected through a grid resistor 69 tothe cathode 'll of the second firing valve. Consequently, when theresultant potential in the circuit consisting of the main capacitor 3,the primary winding 9 and the ignitrons 33 and 35, has the oppositepolarity from the original charge on the main capacitor, the firingvalve 6! becomes conductive to render the ignitron 35 conductive.

The main capacitor 3 is originally charged through charging circuit 1 toa voltage selected by adjustment of the capacitor to effect welding ofthe material 59 upon the first discharge of the capacitor through theprimary winding 9 and the ignitron 33. Because of the characteristics ofthe circuit, the subsequent inverse charge on the main capacitor 3 isnot of the same magnitude as the original charge but is sufiicientlylower that the discharge thereof through the primary winding is notsufficient to effect welding. The discharge of the inverse charge on thecapacitor does, however, act to return the flux of the weldingtransformer a considerable. way toward its original state. Moreover, theportion. of the inverse charge which is not employed in resetting theflux is again stored in the capacitor.

3 with the original polarity so that. it. is not wasted.

To completely reset the flux, a flux resetting. circuit '53 including afull wave rectifier circuit.

energized from the alternating supply lines. 5 and 6 is provided tosupply direct current through a portion of the primary winding 9 in adirection opposite to the direction of current fiow during the firstdischarge of the main capacitor 3 so long as a contactor Bl of a backpressure switch 86 is closed. This contactor 8| is held open throughoutthe welding period.

An air pressure system is associated with one of the welding electrodesit: which is shown as movable with respect to the other electrode IT.The movable electrode l5 has a piston 14 mounted on the upper-endthereof within an air pressure cylinder 75. A pair of air lines l6 andTi are arranged so that one line '16 supplies air under pressure above.the piston l5 tending to lower the electrode l5 and the other line 17supplies air under pressure below the piston 75 tending to raise theelectrode l5. The supply of air to the cylinder 75 through air line itis under the control of a valve member 99, the valves 9'! and 98 ofwhich are operated by a solenoid 90. Valve 91 controls communicationbetween air line it and an exhaust port 73 while valve 98 controlscommunicationb-etween air line 76 and an air supply line E9 which is asource of air under a predetermined pressure. Originally, the air Valves9i and 98 are in the positions illustrated with valve 91 open and valve98 closed;

The control of the air supply to the cylinder '65 through air line ll isunder the control of another valve member H33, the valves IGI and E62 ofwhich are operated by a solenoid I00. Air valve it! controls the supplyof air from another air line I which is a source of air under apredetermined pressure less than the pressure available through air line79. Air valve I02 controls communication between air line 71 and anexhaust port ml. Originally air valves NH and I02 are in the positionillustrated with valve I'Ul open and valve Hi2 closed. Consequently,electrode i5 is originally in the raised position.

In making a weld, air valves 9'! and 98 are first operated so that airis supplied through air line 16 to the cylinder l5 above the piston 14at a pressure greater than the pressure exerted below the piston by airthrough line 11. As a result the electrode I5 is moved downwardly toclamp the material I9 to be welded between the two electrodes I5 and I1with a pressure equal to the difference between the pressure throughline 16 and line 11. Just before the downward pressure on the materialbecomes stabilized the spring biased piston 83 in the back pressureswitch 89 is moved downwardly by back pressure in air line to open thecontactors 8| and 82 carried thereby. The system is now in condition forthe welding period in which welding current is supplied through theelectrodes and the material therebetween. At a preselected instantduring the fiow of welding current, the valves IOI and I02 are operatedto remove the pressure applied from the supply line 11, and the airbelow the piston 14 is exhausted through the exhaust port I 91. As aresult the pressure exerted by the electrodes on the material is rapidlyincreased by a predetermined amount to eflect forging. Upon theconclusion of the weld, the valves 91 and 98 and HH and I92 are againreturned to their original positions to raise the electrode I5 andpermit the material I9 to be moved to a new position. It is to beunderstood that the detailed air pressure system described is merelyillustrative of one of several systems which may be used.

A novel timing system is provided in accordance with my invention torender the first firing valve 4| conductive at a particular instant andto energize the solenoid I90 to operate valves NH and I02 at anotherparticular instant, whereby to effect a weld with a forging operation.The timing system includes a first electric dischar e control valve I09of the arc-like type, preferably a thyratron, having its anode III andcathode H3 connected in series with a second capacitor I I5 and theprimary winding I I1 of the auxiliary transformer 53, the secondarywinding 5| of which is in the control circuit of the first firing valve4|. The second capacitor H5 is originally connected in a circuit whichextends from one side of the secondary winding H9 of another auxiliarytransformer I2I, energized from the alternating current supply lines 5and 6, through a dry rectifier I 23, a normally closed contactor 94 andthe capacitor I 15 to the other side of the seecondary winding H9. It ito be noted that charging of both the main capacitor 3 and the secondcapacitor I I5 is begun as soon as the supply lines 5 and 6 areenergized.

The control circuit of the first control valve I99 may be traced fromthe cathode I I3 through a conductor I to an intermediate tap I21 on avoltage divider I29, another conductor I3I, a third capacitor I33, stillanother conductor I and a resistor 31 to the control grid I39 of thevalve. The voltage divider I29 has a regulated direct current voltagethereacross which is provided from the alternating current supply lines5 and 6 through a voltage regulator MI, an auxiliary transformer I43, afull wave rectifier I and filtering elements I41. The polarity of thevoltage across the voltage divider I29 is such that it tends to-maintainthe control grid I39 of the first control valve I09 negative withrespect to its cathode H3 to maintain the control valve non-conductive.The third capacitor I33 is originally in a discharged state because ofthe discharge circuit completed thereacross through a normally closedcontactor 9I and a resistor I49.

The timing system also includes a second electric discharge controlvalve l5I of the arc-like type, preferably a thyratron, having its anodeI53 and cathode I55 connected in series with the operating coil I94 ofthe solenoid I98 through a normally open contactor 95 and a resistor I51across an auxiliary source of direct current potential I59 energizedfrom the alternating current supply lines 5 and 6. The control circuitof the second control valve I5I may be traced from the cathode I55through the conductor I25 to the intermediate tap I21 on the voltagedivider I29, another conductor I3l, a fourth capacitor I6I, stillanother conductor I53 and resistor I95 to the control grid I91 of thesecond control valve. The voltage supplied in the control circuit by thevoltage divider I29 tends to maintain the control grid I61 negative withrespect to its cathode I55 to maintain the second control valve non-con'ductive. The fourth capacitor I9I is originally in a discharged statebecause of the discharge circuit connected thereacross through thenormally closed contactor 93 and a resistor I39.

An adjustable resistor ill is connected in series with the thirdcapacitor I33 and another adjustable resistor 113 is in series with thefourth capacitor I9I. These series connected capacitor and resistorarrangements are in parallel circuit relation with the other and inseries with the anode I15 and cathode I11 of an electric dischargedevice I19 of the arc-like type, preferably a thyratlon, and a normallyoriginally open contactor 92 across the end terminals of the voltagedivider I29. A large stabilizing resistor I8I is also connected inparallel with the series connected capacitors and resistors IN and I13,I33 and HI.

The control circuit of the discharge device I19 may be traced from itscathode I11 through a resistor I83, the secondary winding i of atransformer I81 and a grid resistor I89 to the control grid I9I of thedevice. A biasing potential app-ears across the resistor I93 asdeveloped by current supplied from the alternating current supply lines5 and 6 through the auxiliary transformer I43, 2. dry rectifier I 93 anda capacitor I95 connected in shunt with the resistor I83. This biasingpotential tends to make the control grid I9I negative with respect tothe cathode I11 and so maintain the discharge device nonconductive.

The primary winding Iil1 of the transformer I81 is connected in serieswith a resistor I99 and a normally closed contactor 82 of the backpressure switch 89 across the voltage divider I29. Another resistor 29Iis also connected directly in shunt across the primary winding 91. Thusthe direct current voltage builds up the magnetic flux in thetransformer I81 so that when the contactor 82 is opened the decay offlux in the transformer produces a voltage impulse across the secondarywinding I85 of such polarity and magnitude as to overcome the biasingpotential and render the discharge device Ii9 conductive To initiate awelding operation, a switch 293 is closed, completing a circuit from thealternating current supply lines 5 and 9 through the operating coil 295of the solenoid 90. This switch 293 may be closed by hand, to be openedas the weld is completed, or may be closed and opened automatically andperiodically by any suitable timing system, such as that disclosed inthe copending application of Pearson and Faulk, Serial No. 442,939,filed May 14, 1942 and assigned to the Westinghouse Electric 8;Manufacturing Company. Such automatic and periodic operation would, ofcourse, result in a series of successive welding operations,

While the solenoid as is illustrated as operating the contactors iii,92, 93, 9 5, 95 and 95 as well as the valves ti and 93, it is apparentthat separate solenoids may be employed if desired.

When the operating coil 2% is energized, the first contactor 9! opensthe discharge circuit across the third capacitor ltd; the secondcontactor 92 closes the anode circuit of the discharge device H9; thethird contactor 93 opens the discharge circuit across the fourthcapacitor Nil; the fourth contactor 94 opens the charging circuit of thesecond capacitor M; the fifth contactor 95 completes the anode circuitof the second control valve I'El and the sixth contactor 96 completes acircuit by which a direct current biasing potential is provided from anauxiliary source across a resistor 258 which is connected in circuitbetween the grids 2M and H3 and oath odes 2l5 and 2!! of the rectifiervalves 23 and 25. The application of biasing potential across theresistor Zilii serves to prevent the rectifier valves 23 and 25 frombecoming conductive and thus halts the how of charging current throughthe main capacitor 3,

Energization of the operating coil 285 also operates the valves 9? and93 in the valve member 99 to apply pressure above the piston it, movingthe electrode is downwardly to clamp the material l9 in place. As thematerial is clamped in place with the desired pressure, the piston 83 ofthe back pressure switch 88 is moved downwardly. The first contactor 8!of the back pressure switch 89 then opens the flux resetting circuit l3and the second contactor 82 opens the circuit through the primarywinding IQ! of the transformer I81. As previously indicated, when thecircuit through the primary winding 19? is opened, a voltage impulse isproduced across the secondary winding I 85 which overcomes the biasingpotential and renders the discharge device I19 conductive.

When the discharge device H9 becomes conductive, current flows from thevoltage divider E29 through the device H9 and the adjustable resistorsIll and US to charge the third and fourth capacitors I33 and H6! at arate determined by the adjustment of the resistors ill and H3. Thepolarity of th charge on the third capacitor I33 is such that when thecapacitor charge reaches a predetermined voltage, the biasing potentialprovided in the control circuit of the first control valve I99 isovercome, causing the control grid 139 to become positive with respectto the cathode H3 to render the control valve 559 conductive. When thefirst control valve 109 becomes conductive, the second capacitor H5 isdischarged through the primary winding ill of the transformer 53 tosupply an impulse in the control circuit of the first firing valve M andthereby initiate the flow of welding current as described.

The polarity of the charge across the fourth capacitor I6! is such thatwhen a predetermined voltage is reached across the capacitor, thebiasing potential supplied from the voltage divider 529 in the controlcircuit of the second control valve I5! is overcome, causing the controlgrid 67 to become positive with respect to the cathode l55 to render thesecond control valve conductive. When the second control valve becomesconductive, current flows through the valve and the operating coil Hi lof the second solenoid Hill to energize it.

It .is tobe noted that since both the third and fourth capacitors E33and IEI are charged by current flowing through the single dischargedevice Ill'i, the charging of the capacitors may be initiatedsimultaneously. Because of the adjustable resistors il'l and l'l3associated with the capacitors let and Hit, respectively and the use ofa regulated direct current voltage source for charging the capacitors,each capacitor may be charged to the voltage necessary to render theassociated control valve conductive at an exactly predetermined instant.Moreover by providing separate sources of potential for charging. thethird and fourth capacitors and for energizing the operating coil of thesolenoid 153i and the transformer 53, good regulation of the voltageacross the voltage divider I29 may be maintained throughout to provideaccuratecharging rates.

A typical cycle of operation is shown by the curves of Fig. 2 in whichthe broken line curve 25% represents the electrode pressure and thefull-line curve 2 ll! represents the current through the primary 9 ofthe welding transformer. Let it be assumed that the switch .2113 isclosedat the point 2i! to energize coil 2% of solenoid 90. After a shorttime delay the first solenoid 9? is operatedand the electrodes l5 and Hare closed at the point 253. Electrode pressure builds up graduallyalong line M5 to the desired welding pressure at 2 l I. Just before thefull welding pressure is reached, the back pressure switch is operatedas at point 299 to initiate charging of the third and fourth capacitorsI33 and Nil. The fourth capacitor I65 may be charged sufiici ntly torender the second control valve [5| conductive at the point'22l. Thiscontrol valve 15! energizes the operating coil IM of the seeone,solenoid I98 which does not effect opening of the valves it! and H12until a predetermined time later. At the point 223 the third capacitor533 is charged sufficiently to render the first control valve [89conductive to initiate the flow of welding current as illustrated bycurv 2E0. It is to be noted that the first portion of curve 2H1 which isabove the base line represents the initial discharge of the maincapacitor 3 and the second portion below the base line represents thedischarge of the inverse charge on the capacitor.

At the point 221 the solenoid its which was energized at point 22!operates valves Iili and H32 so that the electrode pressure increasesrapidly along line 229 to the desired forging pressure levelat 23 l.Current, through the welding transformer ceases at the point 233 and theswitch 203 may be opened shortly thereafter as at point 234. Opening ofswitch 203 deenergizes coil 205 of solenoid 9iiso that valves 9? and 98are opened at point 235 to remove the air pressure above the piston ll.Deenergization of coil 265 of solenoid 96 also causes contactors 9| and93 to close the discharge circuits across capacitors I33 and NH,contactor 92 to open the anode circuit of discharge device I78,contactor $4 to close the charging circuit for the second capacitor H5,contactor $5 to open the circuit through coil l04 of solenoid Hi3, andcontactor 96 to open the blocking potential circuit permittingrecharging of the main capacitor. Deenergization of coil Hi l of thesolenoid Ida causes pressure to be applied through valve lill below thepiston 74 to raise the electrode l5 at point 237.

The time frompoint M9 to point 223 is determined by the rate of chargingof the third capacitor I33. The time from point 2E9 to point 221 is madeup of the adjustable time from point 2 H! to point 22!, determi ed bythe charging rate of the fourth capacitor IBI', and the constant valveoperating or delay time from point 22! to point 221. Thus, the positionof point 221 with respect to the current curve 210 may be accuratelyadjusted over a wide range by means of resistors Hi and H3.

As previously indicated the entire Welding operation may be repeated byreclo ing switch 203 after capacitors 3 and H5 are recharged.

Although I have shown and described a preferred embodiment of myinvention, I am aware that many modifications thereof are possiblewithin the spirit of the invention. I do not intend therefore to limitmy invention to the specific embodiment disclosed.

I claim as my invention:

1. For use with a resistance welding machine having a transformer with aprimary and a secondary winding, the terminals of the secondary windingbeing individually connected to difierent ones of a pair of weldingelectrodes with which a fluid pressure system is associated foreffecting relative movement of the electrodes to clamp material to bewelded thereb tween with a predetermined pressure and including asolenoid valve operable to increase the pressure by a predeterminedamount. the combination comprising power sup ly means including first,normally nonconductive, electric valve means, adapted to be operable tosupply a pulse of current through said primary winding when said firstelectric valve means is rendered conductive, first circuit meansincluding a second normally non-conductive electric valve, adapted toenergize said solenoid valve when said second electric valve is renderedconductive, an electric discharge device, a pair of capacitors connectedin parallel circuit relation with respect to each other and in serieswith said discharge device, second circuit means adapted to impress avoltage across said series connected discharge device and paralleledcapaci tors. means for rendering said discharge device conductive toinitiate charging of said capacitors simultaneously, first control meansresponsive to a first predetermined voltage on one of said capacitorsfor rendering said first electric valve means conductive, and secondcontrol means responsive to a second predetermined voltage on the othercapacitor for rendering said second electric valve conductive.

2. A resistance welder comprising a pair of welding electrodes. atransformer having a prirnary and a secondary winding with the terminalsof the secondary winding individually connected to different ones ofsaid welding electrodes, a fluid pressure system associated with saidelectrodes for efiecting relative movement thereof to clamp the materialto be welded therebetween with a predetermined pressure, a solenoidvalve in said fluid system operable to increase the pressure exerted onthe material by a predetermined amount, power supply means includingfirst normally non-conductive electric valve means, adapted to beoperable to supply a pulse of current through said primary winding whensaid first electric valve means is rendered conductive, first circuitmeans including a second normally nonconductive electric valve, adaptedto energize said solenoid valve when said second electric valve isrendered conductive, an electric discharge device, a pair of capacitorsconnected in parallel circuit relation with each other and in serieswith said discharge device, second circuit means adapted to impress avoltage across said series connected discharge device and paralleledcapacitors, means for rendering said discharge device conductive toinitiate charging of said capacitors simultaneously, first control meansresponsive to a first predetermined voltage on one of said capacitorsfor rendering said first electric valve means conductive, and secondcontrol means responsive to a second predetermined voltage on the othercapacitor for rendering said second electric valve conductive.

3. For use with resistance welding machine having a transformer with aprimary and a secondary winding, the terminals of the secondary windingbeing individually connected to different ones of a pair of weldingelectrodes with which a fluid pressure system is associated foreifecting relative movement of the electrodes to clamp the material tobe welded therebetween with a predetermined pressure and including asolenoid valve operable to increase the pressure by a predeterminedamount, the combination comprising pow er supply means including firstnormally nonconductive electric valve means of the arc-like type,adapted to be operable to supply a pulse of current through said primarywinding when said first electric valve means is rendered conductive,first circuit means including a second normally non-conductive electricvalve of the arc-like type, adapted to energize said solenoid valve whensaid second electric valve is rendered conductive, an electric dischargedevice of the arc-like type, a pair of capacitors, an adjustableresistor in series with each of said capacitors, each series connectedcapacitor and resistor being connected in parallel circuit relation withthe other and in series with said discharge device, second circuit meansadapted to impress a voltage across said series connected dischargedevice and paralleled capacitors and resistors, means for rendering saiddischarge device conductive to initiate charging of said capacitorssimultaneously, the rate of charging of each of said capacitors beingdeterminable by the adjustment of the corresponding resistor, firstcontrol means responsive to a first predetermined voltage on one of saidcapacitors for rendering said first electric valve means conductive, andsecond control means responsive to a second predetermined voltage on theother capacitor for rendering said second electric valve conductive.

HARRY J. BICHSEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,327,268 Jenks Aug. 17, 1943

